Preparation of diffusion coatings on metals



May 19, 1959 1 w. KOCH 2,837,407

Y "PREPARATION-OF DIFFUSION COATINGS ON METALS Filed Aug. 5, 1957 INVENTOR WALTER KOCH BY MC/(Y ATTORNEYS United States Patent I PREPARATION OFDIFFUSION COATINGS ON METALS Walter Koch, Duesseldorf, Germany, assignorto Manufacturers Chemical Corporation, New York, N.Y., a corporation ofNew York This invention relates to the preparation of diffusion coatingson iron and other metals and it has particular relation to thepreparation of such coatings by reacting the metal to be coated withgaseous halogen compounds of suitable other metals.

It is an object of the present invention to provide a process and devicefor producing metallic diffusion coatings of high quality and uniformstrength on articles consisting of iron, steel or other base metals,including articles having difficultly accessible parts.

It has been known that gaseous halogen compounds of suitable metalschemically react with solid iron and certain other base metals atelevated temperatures, forming gaseous halides of iron or said othermetals and depositing the metallic element of the gaseous halogencompound or compounds used as starting material, in finest subdivisionon the surface of iron or said other base metals. If the metallicelement deposited or precipitated from the gaseous starting halogenideis capable of diffusion into the base metal at the temperature used, itwill penetrate into the base metal, while atoms of the base metal willbe brought from interior layers to the surface of the latter likewise bydiffusion. The chemical reaction of the surface of iron, or said otherbase metals, can be carried on or continued by causing further amountsof the gaseous metal halide to come in contact with the iron or otherbase metal at diffusion temperature and removing the iron or other basemetal halide formed in the reaction, from the iron or other base metal.

It has been found that in order to obtain high-grade,

- dense surface layers having the desired exact dimensions,

on iron or other base metals, by the above mentioned chemical reactionbetween gaseous metal halides and solid iron or other base metals, it isessential that the gaseous metal halide or halides used as startingmaterial be uniformly applied to all surface portions of the iron orother metal articles to be coated, i. e. at a uniform concentration ofthe gaseous halides in the reaction gas. Furthermore, the volume ofmetal deposited on iron or other base metal by said chemical reactionshould be about equal to the volume of metallic iron, or other basemetal, converted into and removed from the latter in the form of gaseoushalidg It has been found that said uniform application of the gaseoushalide or halides requires the use of gaseous halides which do notcontain admixtures, diluents or carriers. Owing to differences inspecific gravity, the gaseous metal halides do not mix completelyuniformly with carriers or diluents, e.g. inert gaseous carriers, and ifthe latter are used, uniformity of the surface coating is adverselyaffected. This also happens if a mixture containing gaseous and liquidand/ or solid metallic halides is applied. If no other than gaseousmetal halides are used, i.e. if the reaction gas consists of gaseousmetal halides only, said lack of uniformity of the coating can beavoided even in the treatment of difiicultly accessible surfaces ofmetallic articles, e.g. surfaces of long and narrow bores.

However, the composition of the reaction gas consisting of gaseousmetallic halides only, must be adjustable in order to adapt the reactiongas to the particular base metal on which the diffusion coating issupposed to be formed.

According to the present invention, the above requirements are met byplacing the articles to be provided with a diffusion coating in afurnace which is connected during treatment with a second furnace and athird furnace interconnected to said second furnace, evacuating thethree furnaces, producing the desired reaction gas to be used forcoating said articles, in said second and third furnaces and regulatingthe composition of the reaction gas by the adjustment of the temperaturein said second furnace and the temperature in the third furnace.

Thus, the main object of the present invention is to provide a processand a device and arrangement for providing metallic articles with ametallic diffusion coating of high quality by means of a reaction gasconsisting of gaseous metallic halides, whereby the desired compositionof the reaction gas is brought about by regulation of the temperature insaid second furnace and of the temperature in said third furnace.

Other objects and the advantages of the invention will be apparent fromthe following specification and the appended claims and drawing, whichdisclose by way of example and without limitation some specificembodiments of and best ways for carrying out the invention.

In the embodiment shown in the drawing, articles con- 'sisting of thebase metal, for example iron, are treated I with a gaseous metal halidein a tubular furnace generally denoted by reference numeral 1. Thisfurnace consists of highly heat-resistant steel and can be operatedunder ordinary atmospheric pressure, or excess pressure, or undervacuum. If it is desired to evacuate the furnace, a suction device orblower can be attached to pipe means 23 extending from the upper portionof the furnace. One end of furnace 1 is sealed by a cover 4, providedwith cooling means 5, cooled, for example, by water, while its other end6 is tightly connected with outlet 7 of a second tubular furnacegenerally denoted 2. Furnace 2 is tightly connected by its tubularextension 8 to tubular outlet 9 of a third furnace generally denoted 3.Thus, the three furnaces 1, 2, 3 are connected in series. They can beheated in any suitable manner, for example by immersion in heated saltsolution, electric heating means, etc., not shown in the drawing.Furnace 2 is preferably provided with a ceramic lining 10, while furnace3 can be made entirely of ceramic material, if desired. The threefurnaces, 1, 2, and 3, are tightly connected with each other and theirtemperatures can be regulated by said heating means, separately andindependently in each furnace. Furnace 3 may contain pieces 22 of asolid material arranged on a support. This solid material serves forpreventing the entrance of liquid halide from furnace 3 into furnace 2,in the case of bumping of the boiling halide in furnace 3, and itconsists for example of the metal present in furnace 2 or of someceramic material. 5a denotes a ribbed insert, the temperature of whichincreases from that of cooled member 5 up to the temperature in theinterior of furnace 1 and in the cooler ribs of which condensed halides,e.g. of iron are often deposited.

Example 1 If articles of iron should be provided with a diffusioncoating of titanium, furnace 3 is charged with liquid titanium chloride(TiCl as indicated at 11, while furnace 2 contains metallic titanium 12,in the form of pieces having a sufiiciently large. surface, e.g. pieceshaving an average size of 1 to 10 mm. The metallic 12 rests on aperforated plate 21 arranged in furnace 2, as shown in the drawing. Inorder to remove the-air from the three furnaces, the system is firstevacuated through a valve arranged preferably at the cover of furnace 1,the liquid titanium tetrachloride in furnace 3 being kept cold duringthis evacuation. After evacuation, there will bea pressure correspondingto the vapor tension of titanium chloride, in all three furnaces. Aslong as the titanium chloride is cold, this pressure is substantiallybelow ordinary atmospheric pressure. Furnace 2 is now heated first to atemperature of for example about 400 C., at which thetitaniumtetrachloride vapors react with the metallic titanium 12, with theformation of titanium trichloride (TiCl so that the gas entering furnace1 consists of TiCl Furthermore, furnace ,1 which contains the ironarticles to be treated (not shown in the drawing) is now heated'to atemperature in the range of 700 to 1300 C., at which reaction of TiClwith iron and diifusion of metallic titanium into the iron articles cantake place, while furnace 3 is simultaneously heated to l150 C. in orderto bring about the desired T iCl vapor pressure which can be higher orlower than ordinary atmospheric pressure in thercase of the presentexample. During this coating process, the temperature in furnace 2 iskept in the range of 400 to 700 C.

The gaseous titanium chloride (TiCl reacts with the iron articles infurnace 1 with the formation of gaseous ferrous chloride (FeCI which iscondensed and collected on the cooled surfaces 5 and in the uppercooling ribs 5a of furnace 1. After suflicient titanium has beendeposited in the treated iron articles, furnaces 1 and 3 are firstpermitted to become cool. By conducting the process for about 240minutes, a titanium diffusion coating of 0.03-0.10 mm. thickness isobtained.

It is of particular advantage to operate furnace 1 at a high temperatureat the beginning of the treatment and at a lower temperature at the endof the treatment. For example, the coating treatment is carried out for240 minutes and the high temperature is applied for about 200 minutesand the low temperature for about 40 minutes. It has been found that byproceeding in this manner a particularly high concentration of the metalto be deposited in the base metal, can be attained in the top surfacelayer. In the treatment of iron and steel base metals in furnace 1,temperatures in the range of 1050 to 1300 C. can be used as highdiffusion temperatures and the range of 700 to 1050 C. can be used forlow diffusion temperatures. The same temperatures can be used in thetreatment of hard metal alloys, e.g. metals having a tungsten carbide ortitanium carbide-cobalt basis. These alloys can be treated also beforebeing sintered. Thus the formation of diifusion coating and sinteringcan take place in the same step.

By maintaining the above described temperatures in furnaces 1, 2, and 3,respectively, the formation of a reaction gas consisting of TiCl and theformation of very satisfactory titanium diffusion coatingson the ironarticles is obtained.

Example 2 In order to obtain a uniform, dense and durable diffusioncoating of chromium, the reaction gas must substantially consist of CrClvapor, because no uniform, satisfactory diffusion coatings are formed ifthere is substantial excess of Cl or Cr (in comparison with the.

formula of CrCl in the reaction gas.

The articles to be coated-cg. threaded pieces consisting of commercialsoft ironwere placed in furnace 1 and the air was removed from the threeconnected furnaces by evacuation. Furnace 2 contained about 500 grams ofchromium metal in small pieces and furnace 3 about 275 grams of CrClFurnace 1 was heated to about 1150 C., furnace 2. to about 1200" C., andthe temperature in furnace 3 was kept at about 1000-1100 C. The pressurein the furnace system was about 30 Torr and the treatment was continuedfor about two Example 3 In carrying out the present invention, diffusioncoatings comprising. several metals, such as coatings oftitanum+chromium, or silicon-i-chromium, or vanadium-l-chromiurn, provedto be of particular interest.

Such coatings can be, for example, prepared in a one step process byfirst. depositing silicon. or titanium at a relatively high pressure andsubsequently depositing chromium at a relatively low pressure, i.e.below mm. Hg, by cooling, furnace 3 to the necessary extent. As a resultof this treatment, a thickchromium diffusion layer of 0.35-0.5 mm. isformed in about four hours, while in the application of chromium alone,under otherwise equal conditions, a chromium difiusion layer ofabout.0.1 mm. only, is obtained.

In order to prepare a chromium-silicon, diffusion coating on iron, acontainer placed in furnace 3 is charged with 1 part by weight ofsilicon tetrachloride (SiCl and 3 parts by weight of chromous chloride(CrCl to which 1 part by weight of chromium metal powder is added.Furnace 2 is charged with relatively coarse grained chromium metal, asmentioned in Example 2. The connected furnaces, are evacuated as in theprevious examples and thetemperature is maintained at first in furnace 1in the range of 900 to 1100 C.; in furnace Z in the range of 300 to 500C. and in furnace 3 in the range of 20 to 60 C. The treatment iscontinued for about 60 minutes. After this period the silicontetrachloride is\ consumed, and there is a low pressure in the furnace;Then furnaces 1 to 3 are heated as mentioned for chromium diffusioncoatings, i.e. furnace 1 to about 1150 C., furnace 2 to about 1200 C.,and furnace 3 to about 1000-1100 C., whereby a diffusion coating of 0.2to 0.5 mm. is obtained.

Example 4 In order to produce on iron a diffusion coating of chromiumalloyed with vanadium, furnace 3 is charged with 300 grams of a mixtureconsisting of 3 parts by weight of CrCl powder and 1 part by weight ofmetallic vanadium powder and furnace 2 is charged with a mixture. of 3parts by weight of metallic chromium (grained) powder and 1 part byweight of metallic vanadium. (grained), while into furnace 1, in whichthe parts to be coated are placed, 1 part by weight of freshly preparedCrCl is introduced. The connected furnaces are evacuated as in theprevious examples and the temperature is maintained in furnace 1 in therange of 1050" to 1150 C.; in. furnace-2 intherange of 1l00to 1200 C.and in furnace'3 in the range of 1000 to 1100 C. The treatment iscontinued for about 240 minutes under a pressure of 30 Torr, whereby adiffusion coating. of about 0.15 mm. is obtained.

Example5 In order to prepare a titanium diffusion coating on iron,furnace 3 is charged with 1 part by Weight of T iCL; and furnace 2 ischarged with 2 parts by weight of titanium metal. The connected furnacesare evacuated as in the previous examples and the temperature ismaintained in furnace 1 in the range of.700 to 1300 C.; in furnace 2 inthe range of 400 to 700 C. and in furnace 3 in the range of 100 to C.The treatment is continued for about 240 minutes under apressure of500-1500 Torr, whereby a diffusion coating of 0.03-0.8 mm. is obtained.

The pressure to be used in the above described process and devicedepends on the circumstances of each individual case, and may 'beordinary atmospheric, or 'subatmospheric, or superatmospheric pressure.In general, a pressure in the range of 300-1000 mm. Hg is desirable.Under higher pressure the reaction velocity is higher than under lowerpressure, other conditions being equal. If a deep penetration of themetal to be deposited at low concentration is desired, a relatively lowpressure can be of advantage. invention is in practice often limited bythe vapor pressure of the halides used and also by the material of thefurnaces. If the furnaces are operated under high thermal strain, theuse of high pressure or high vacuum is prefera-bly avoided. Thus, theoptimum pressure to be used is determined, on the one hand, by thefundamental desirability of high pressure and, on the other hand, by thespecific conditions of the individual case.

The specific halide of the metal to be used in coating iron or otherbase metal, should be selected to comply with the requirement ofdepositing by chemical reaction of the halide with the base metal, avolume of the coating metal which is substantially equal, or nearlyequal, to the volume of the base metal converted into halide by saidchemical reaction. Thereby, instead of one halide, a mixture of two orseveral halides of the coating metal or metals can be used.

As an example of carrying the above described principle into effect, thepreparation of a titanium difiusion coating on iron is mentioned.

The molecular volume of titanium is about 10.8, while the molecularvolume of iron is about 7.1. All chlorides of titanium-the tetrachlorideTiCl as well as the lower chlorides-form ferrous chloride (Fecl in thereaction with iron. Therefore, an optimum effect will be obtained bydepositing 2 molecular volumes of titanium, while removing 3 molecularvolumes of iron from the iron base metal according to the equation:

Instead of 2TiCl a mixture of TiCI and TiCl containing substantially thesame amount of Ti as TiCl can be used. Furthermore, instead of using asingle metal, a mixture of halides of more than one metal, for example amixture of TiCl and ZrCl can be used. But in this case too, the sum ofvolumes of the metals deposited, i.e. Ti and Zr, should besubstantially, or nearly, equal to the volume of Fe removed in the formof FeCl from the base metal. Instead of chlorides, other halides, e.'g.fluorides, or mixtures of various halides can be used.

The invention can be used in the preparation of diffusion coatings onarticles consisting of various metals or of mixtures or alloys of two ormore metals. The invention is particularly suitable for the preparationon steel and iron of diffusion coatings consisting of metals or otherelements which are capable of diffusion in iron and form easily volatilehalides, particularly chlorides, such as, for example, silicon,titanium, zirconium, uranium, vanadium, niobium, boron, aluminum,arsenic, beryllium, germanium, copper, nickel, phosphorus, rhenium,sulfur, selenium, tantalum, tellurium, thorium, thallium, etc.chlorides, the boiling and/or sublimation temperatures of which arelower than those of ferrous chloride (FeCl However, instead of thesechlorides, or in mixture with them, other chlorides having boilingand/or sublimation temperatures which are higher than those of FeCl suchas for example CrCl can also be used. In using the latter, or othermetal halides having such higher boiling and/or sublimationtemperatures, the present process should be carried out under relativelylow pressures, if necessary under the continuous action of a pumpconnected with the system of furnaces 1, 2 and 3. Furthermore, in suchcases part of the chlorides can be introduced directly into furnace 1.

The present invention can be applied with particular advantage tosintered and hard metals, the pores of which become filled with thedeposited metal to the desired The pressure used in carrying out thepresent 6 extent and the density of which is increased by the process ofthe invention. Articles of sintered iron and of sintered hard metals canbe provided with diifusion coatings in this manner. In many cases,sintering and production of diffusion coatings on the articles, can takeplace in the same step in furnace 1.

If articles of steel are treated according to the present process withmetals having high aflinity to carbon, difiusion of said metals intosteel is facilitated if in the articles to be treated the carbon of thesteel has been previously combined with elements which easily formcarbides, such as titanium, vanadium, zirconium, molybdenum, tungsten,or excess chromium or manganese.

The CrCl powder used in carrying out the invention is preferablyprepared by the reduction of chromium chloride with hydrogen. It ishighly hygroscopic and should be very carefully protected from moisture.It can be also prepared by the action of chlorine and of hydrogenchloride on metallic chromium, or by the action of mixtures of chlorineand hydrogen chloride on metallic chromium, directly in liquid state atabout 850-900" C.

The average duration of the present process amounts to 2-4 hours whenthe system is heated to the necessary temperature, i.e. this period doesnot include the time necessary for heating the furnaces to the reactiontemperature and for cooling.

The term base metal is used in the present specification and claims toinclude metals or alloys which are subjected to the process of thepresent invention by reacting them with halides of metals to bedeposited on the base metals. The term diffusion coating is used todenote surface layers obtained by subjecting base metals to the processof this invention in order to deposit other metals on the base metals attemperatures, at which the deposited metals penetrate into the basemetals by diffusion, and diffusion temperature denotes a temperature atwhich such difiusion occurs.

It will be understood that this invention is not limited to the specificsteps, materials, construction and other specific details describedabove and can be carried out with various modifications. For example,base metals other than those described above can be used and as examplesof such other base metals tungsten, nickel and sintered hard metals,i.e. metals having a tungsten carbide-, or titanium carbide-cobalt-basisare mentioned. Hard metals can be provided, for example, with adiffusion coating of chromium or titanium in a manner substantiallysimilar to that described above in connection with base metals or ironor steel. Corrosion resistant surface coatings on hard metals can bethus obtained. Furnace 1 can be charged with articles, the sintering ofwhich has not been completed yet. The depth of penetration is relativelyhigh in this procedure. Sintering can be carried out simultaneously withformation of the coating or subsequently.

Reference is made to my co-pending patent application filed under SerialNumber 236,678 on July 13, 1951, and now abandoned, of which this is acontinuation-inpart.

What is claimed is:

1. In a process for diffusion coating a base metal with a coating metalby halide substitution, the steps comprising positioning said base metalin a first furnace, positioning said coating metal in a second furnacecommunicating with said first furnace, positioning a halide of saidcoating metal in a third furnace communicating With said second furnace,evacuating said three furnaces, applying heat to said third furnace tovaporize said halide, applying heat to said second furnace to react saidhalide with said coating metal to produce a halide reaction product, andapplying heat to said first furnace to react said reaction product withsaid base metal.

2. A process as defined in claim 1 wherein the heat applied to saidsecond furnace is varied to control the chemical composition of saidreaction product.

e sazeov 7 I I A, pmcfisa asi-idefineduin claim; :LIWherein ihe heat' II I I causesaid: second-furnaceto reach the reaction;tempera I I v I Ippliedq; to; iseid thind furnace is varied; to control: the I Ituteofzs'aid coatingjmetal ands-aid Coating metalIhalid i I I ch mic lfimp sifi n fsaid reaction product I 1 I I 1 I I I I :9. 1m processfor'diffusion coatinggabasezmetal with i I I I e I ,4 Azprocess' as;defined; in, claim I 1, wherein the he'at I & coating metal bysubstitutionireaction of a vapor phase I I I fipplieclltosaideecondandihird:furnaces is independently; :ha lide; of: saidcoating'imetal Iwifih said 'ba'semet'al to: tiei 1 ;Yariedt0'controittheehemical composition of said; rear positfsaidcoatingfm'et'al on'saidrbase metal and remove- J O IPIO j I I I I I I, Iv I II I I I :thehalideof said base metal,:the'step comprising va rying'I i y I 5. I A process a defined:- in, claim '1; wherein the:temith'e-Iiratio :of coating metal ito halogen ini said vapor phase I 1i I v I I i I I l P 7 5 0 5 aid fi s ifilllfiafilfi :i i ub an i lly; IBC I I coating metal halide compositiom whereby the volume- I I I I I Iprior :to, completion of said difiusion coating I I I 1(1 of coatingmet'al deposited is substantially equal to the 1 I I I I f 1 :A P aefined inclaiml, wherein th chem- I volume: of base metal: removed; I II I I I I v I I I I I I I I I I I icai composition of, said, reactionproduct- ;is varied by I i I 10. A process: as defined iniclaim 9;wherein the iratio I I I o r i h h PP G a second ird is variedbyreacting said coating metal halide with said i I v i I l I 1 I I i I ffurnacesto produce avolume of deposited :coatingemetal' I I g meta; im nne g n nin e m me; i I :Ia;' I I' b t n lyq a m ew lu of se me alr m ved15 pe'ratu're; atzwhich said reaction takesiplace; I IIII1:IJII

,7 AIIprocess as definedinclaim'l wherein nochem- I I II 3- I I I I 1 II I I I i i'cal element is present in any of'saidfthree furnaces ex w Ri m Pate? I I I I I I I I I I I I 1 I :cept Isaidlbase metal; saidcoating metal and the selected f I I i I 1 U STATES I9ATENTSI I I I I iI I I I I I I I I I I I I I halogenforming a :part: of: saici coatingmetal halide: I i I 1,497,417 I W be i f Jun :19 1924: I I "rims,ompoundyq ii I I 2,401,22151: II 1\ Ia 2s,r1:946i

' I i I I I I I i I i I S; 'Aprocess as-idefinedin claimzhwhereintheamount 2,442,485 l I I I i I i of heat applied to said: secondfurnaee'is sufficient to I l 2 556;?63

2,657,127- I I Sinderbandet'al. Oct; 27; 1953' I I

1. IN A PROCESS FOR DIFFUSION COATING A BASE METAL WITH A COATING METALBY HALIDE SUBSTITUTION, THE STEPS COMPRISING POSITIONING SAID BASE METALIN A FIRST FURNACE, POSITIONING SAID COATING METAL IN A SECOND FURNACECOMMUNICATING WITH SAID FIRST FURNACE, POSITIONING A HALIDE OF SAIDCOATING METAL IN A THIRD FURNACE COMMUNICATING WITH SAID SECOND FURNACE,EVACUATING SAID THREE FURNACES, APPLYING HEAT TO SAID THIRD FURNACE TOVAPORIZE SAID HALIDE, APPLYING HEAT TO SAID SECOND FURNACE TO REACT SAIDHALIDE WITH SAID COATING METAL TO PRODUCE A HALIDE REACTION PRODUCT,